Side pumped optically active components in rod or fiber form are e.g. used as high-power lasers or in communications technology as optical fiber amplifiers with a laser active core and a pump cladding surrounding the core. Apart from lasers, the term “optically active component” also encompasses optical amplifiers and so-called superluminescent sources.
Fiber amplifiers can compensate losses in optical waveguides during optical data transmission in that laser light is injected or coupled into the fiber core by so-called “optical pumping”, with the laser light exciting the core zone. The light pulse running through the fiber core additionally absorbs energy from the excited ions and is thereby amplified.
Alternatively, the laser active substances can also be excited by the injected pump light as such to emit laser light, as is the case in high-power lasers in fiber or rod form.
The laser active material contains, for instance, crystalline Nd:YAG or it consists of quartz glass containing dopants that effect an output or amplification of laser radiation in the host material quartz glass. The dopants are normally rare-earth cations (lanthanoids) or cations of the so-called transition metals.
To ensure an adequate pump light action also after a certain penetration depth, a particularly high energy density of the pump light radiation is needed in the input region. This is accompanied by a degradation of the core region due to heat action and photodarkening. Moreover, the high energy density of the pump light in the input region on the one hand and the exponential decrease in the pump light power on the other hand lead to undesired non-linear effects.
These drawbacks are avoided by side pumped, optically active components and laser systems according to the aforementioned type in which the pump light is not directly coupled into the laser active core, but is coupled via the cladding surface into the core. Due to the length of the fiber the cladding surface is many times greater than the fiber face. This allows the in-coupling of high excitation energy without impairment of the core region of the optically active component.
Such an optically active component in the form of a side pumped laser is known from U.S. Pat. No. 5,048,026 A. Described is a fiber amplifier comprising a cylindrical laser-active core of Nd:YAG and an output end for laser radiation that is surrounded by a sleeve-shaped pump jacket or cladding of quartz glass. The pump cladding is conically shaped in a front longitudinal region, so that its thickness is decreasing towards the output end and terminates in a rear cylindrical part that ends flush with the output end of the core. The pump light is injected into the pump cladding on the face side opposite the output end and is reflected back and forth between the cladding areas of the pump cladding, thereby passing through the laser-active core. To ensure this, the pump cladding has a refractive index greater than the refractive index of the surroundings (e.g. air), but smaller than the refractive index of the laser active core. In the conical part of the pump cladding, the angle relative to the cladding areas is here getting steeper and steeper with each reflection, so that the pump light is focused onto the laser active core in the cylindrical region.
A similar laser system is also known from U.S. Pat. No. 5,086,433 A. The laser system comprises a quartz glass element that serves to optically pump a laser rod which is inserted into a central bore of the quartz glass element. The quartz glass element tapers in propagation direction conically and it is provided with a mirror coating on the outside.
Several laser diodes are used as pump light sources, which radiate at different radial positions on the face side into the quartz glass element. Depending on the radial position of the laser diode, the pump light rays impinge at axially different positions on the mirror coating and are reflected from there into the laser rod.
The pump light injected into the quartz glass element has a substantially annular intensity distribution, and it is also possible, depending on number and position of the laser diodes, to distribute the injected pump light over the length of the laser rod more or less evenly. The device itself and the adjustment of the components are however very complicated.
DE 28 44 129 A1 describes a similar device for optically pumping a laser rod. The device comprises a side pumped Nd:YAG laser and a pump light source. The laser rod is surrounded by a cladding and the latter, in turn, by a sleeve having an inner mirror coating. The cladding consists of a material having a lower refractive index than the laser rod, and it tapers conically in light propagation direction. On the output end a small glass plate is arranged which reflects the pump light and also permits the output or out-coupling of laser light.
The pump light injected by the pump light source from its broad end on the face side into the cladding is reflected at a specific angle on the mirror-coated sleeve and then impinges on the cladding surface of the laser rod.
The manufacture of a laser component with a conically tapering pump light cladding of glass does however pose some problems. Such a component must e.g. be produced in that the pump light cladding is mechanically treated from the outside or in that a cylindrical component is softened zone by zone and elongated in this process, whereby during elongation the drawing rate is continuously increased, so that the outer diameter of the drawn-off strand is continuously decreasing. This procedure is troublesome and requires a complicated control process, and it is particularly not suited for setting a steep cone angle (e.g. more than 10°).
Moreover, a conical pump light cladding will yield a laser component with a tapering outer diameter if no counter-measures are taken. A tapering outer diameter, however, has drawbacks. For instance, cooling over a solid body turns out to be more difficult to realize than cooling at a constant outer diameter, and the customizing of the laser component, particularly a laser fiber, is difficult. Therefore, despite a conical pump cladding, a cylindrical outer cladding of the optical component would be desirable.
WO 2006/049186 A1 discloses a method for producing a preform with a conical core in that first of all a cylindrical preform with a core region and a cladding region, each with a constant diameter, is conically elongated (as explained above), so that a semifinished product is first obtained with a conical core region and a conical cladding region. Subsequently, the outer diameter of the semifinished product is ground at the expense of the cladding region to a constant dimension. The resulting cylinder comprises a conical core region and a reversed conical cladding region.
Grinding entails a lot of work and great loss of material.
JP 11021142 A discloses a different method for producing a cylindrical optical component with a conical core region and a reversed conical inner cladding region. First of all, a cylindrical core rod with a cylindrical core region and a cylindrical inner cladding region is produced. Subsequently, the inner cladding region is removed to obtain a conical shape, resulting in a semifinished product having a conical outer-diameter extension. The conical semifinished product is drawn off in an elongation process to obtain a cylindrical semifinished product with a constant outer diameter. Thereafter, the semifinished product comprises a conical core region and a reversed conical cladding region. The cylindrical semifinished product is subsequently provided with an additional outer cladding and drawn into a fiber.
The known methods require great efforts in terms of work and time for producing a cylindrical outer diameter of the optical component.